A problem that I often see with my undergraduate students is that their tonguing seems to be "mushy" when they are articulating at soft dynamic levels. It often sounds as if they are articulating using a "hnu" syllable, rather than "tu" or "du".
I believe that this problem stems from a habit of trying to just barely brush the tongue against he teeth when tonguing softly. This instinct, I believe, probably is a reaction to previous experiences of having harsh, explosive tonguing in soft passages.
When we play softly, especially if it is a slow passage, I’ve observed that my students often tend to use a slow tongue speed. There’s no reason that in soft or slow passages we should use a slower tongue motion, but I assure you that trumpeters do tend to do this. For example, if we’re tonguing loud quarter notes at mm=120 and our tongue speed is (hypothetically) 100 m.p.h., we will invariably slow the tongue speed to 40 m.ph. if the dynamic changes to piano or the tempo slows to mm=60. As a result, that the tongue, by moving so slowly into the air stream, actually "gets stuck" in the air stream, stops the air momentarily, and then releases with a sfz explosion, which is undesirable and makes the note sound out of context.
After that experience, the player’s reaction is then to be more cautious and try to just brush the tongue against the teeth, in an attempt to avoid that percussive explosion. What the player should be doing, however, is to be more aggressive with the tongue motion, increasing the speed of the tongue into and back out of the air stream, rather than using a slow tongue speed and try to nudge it against the teeth.
It may seem very unnatural, but we must actually think about tonguing HARDER in soft passages and slow passages, rather than tonguing softer or by caressing the tonguing.
You may find it to be beneficial to spend a few minutes playing simple repeated note or scale patterns at quarter note equal to mm=60 at a soft dynamic, and try to "flip" the tongue quickly in/out of the air stream. Try to imagine a "hard" tongue stroke in order to minimize the interruption of the air, and thereby resulting in a smooth, soft articulation.